docs: Start defining the owners of code sections

When filing a new merge request it's often hard to know who to ask for a
review; using the Git log doesn't always help — the person that touched
a file last may just be fixing the build or a compiler warning.

The `CODE-OWNERS` file format is something that GitHub uses in order to
pre-fill the list of reviewers:

  https://help.github.com/articles/about-codeowners/

Ideally, in the future, we'll be able to use this file with a bot like
homu to automatically go through newly filed merge requests and
automatically ask the relevant people for reviews, instead of doing this
manually.

CONTRIBUTING.md

@@ -60,4 +60,6 @@ $ git checkout -b your-branch
 
 Once you've finished working on the bug fix or feature, push the branch
 to the Git repository and open a new merge request, to let the GTK
-maintainers review your contribution.
+maintainers review your contribution. The [CODE-OWNERS](./docs-CODE-OWNERS)
+document contains the list of core contributors to GTK and the areas for
+which they are responsible.

docs/CODE-OWNERS

@@ -0,0 +1,68 @@
+# These are the people responsible for subsystems in GTK; if you're opening
+# a merge request for files listed here, please add the following people to
+# the list of reviewers
+
+# The syntax of this file is similar to the GitHub CODEOWNERS file:
+#   https://help.github.com/articles/about-codeowners/
+# Which, in turn, is similar to the .gitignore and .gitattributes files:
+#
+#  - comments start with `#`
+#  - the first column contains paths and globs
+#  - the second column contains GitLab user names or email addresses,
+#    separated by spaces
+#
+# If you want to be responsible for code reviews in specific sections of
+# the GTK code base, add yourself here.
+
+# Maintainer
+*                       @matthiasc
+
+# Build system
+meson.build             @ebassi @nirbheek
+*.py                    @ebassi
+
+# CSS
+gtk/gtkcss*.[ch]        @otte @baedert
+gtk/gtkstyle*.[ch]      @otte @baedert
+
+# Gestures
+gtk/gtkeventcontroller* @carlosg
+gtk/gtkgesture*.[ch]    @carlosg
+
+# GtkFileChooser
+gtk/gtkfilechooser*     @federico
+gtk/gtkfilesystem*      @federico
+gtk/gtkfilefilter*      @federico
+
+# GtkFontChooser
+gtk/gtkfontchooser*     @matthiasc
+
+# Input methods
+gtk/gtkimcontext*       @carlosg
+
+# Media
+gtk/gtkmedia*           @otte
+
+# GSK
+gsk                     @otte @baedert @ebassi
+
+# GL rendering
+gsk/gl                  @baedert @ebassi
+
+# Vulkan rendering
+gsk/vulkan
+
+# Documentation
+docs/                   @ebassi @dboles
+
+# Wayland
+gdk/wayland             @jadahl
+
+# X11
+gdk/x11                 @ofourdan @matthiasc
+
+# Themes
+gtk/themes              @lapoc @jimmac
+
+# Inspector
+gtk/inspector           @otte @matthiasc